| Electronics > Beginners |
| LED current draw, series / paralell |
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| Gazz292:
i'm probably just being stupid here, but i'm wondering about led current draw, I've been playing about with a sequential turn signal kit for my car, bought a cheapo set off ebay, and the supplied flexible led strips were far too dim for me to put in my car. So i decided to just use the controllers and make my own led boards up, the mosfets used on the controller boards are rated for 2.5 amps, So i got some amber led's (decent ones from RS components, not ebay) they have a forward voltage of 2.1v (max 2.3), and say to use 50ma current for max brightness (6000mcd) The voltage the controllers will see varies from 12 to 14.8 volts, So i put 5 led's in series for each section of the sequential bar, used an online led series/parallel calculator that said for the highest voltage (14.8) use a 100 ohm resistor, and the led's will pull 50ma total from the source. Why only 50ma? i must be missing something, i thought they'd pull 250ma, if only pulling the power of one led, then do i assume that they will be a lot less bright than they could be... but of course running them all singularly off 14.8 volts needs some rather large and hot resistors. |
| Bassman59:
--- Quote from: Gazz292 on May 15, 2018, 07:15:33 pm ---So i put 5 led's in series for each section of the sequential bar, used an online led series/parallel calculator that said for the highest voltage (14.8) use a 100 ohm resistor, and the led's will pull 50ma total from the source. Why only 50ma? i must be missing something, i thought they'd pull 250ma, --- End quote --- All of the LEDs are in series? Kirchoff's Current Law. Follow the current. |
| glarsson:
5 LED at 2.1V is 10.5V. With 14.8V that leaves 4.3 for the 100Ω resistor. 4.3V over 100Ω is 43mA. 18Ω might work for you. Note that the current will vary depending on the "12V". 83mA at 12V up to 238mA at 14.8V. Probably not satisfactory. You have two options. Reduce the number of LEDs in each string so the voltage over the resistor doesn't vary as much or drive the LED strings using a current generator set to the required current. |
| Nerull:
The same current flows through all elements in series. 50mA will flow through each LED. If 250mA flowed through each LED you would just blow them all up. |
| sokoloff:
--- Quote from: Gazz292 on May 15, 2018, 07:15:33 pm ---So i got some amber led's (decent ones from RS components, not ebay) they have a forward voltage of 2.1v (max 2.3), and say to use 50ma current for max brightness (6000mcd) <snip> Why only 50ma? i must be missing something, i thought they'd pull 250ma, if only pulling the power of one led, then do i assume that they will be a lot less bright than they could be... --- End quote --- Current is the same for anything in series (as the above posters indicate). Power for each LED will be given by the voltage drop across the LED times the current (P = E * I). Power for LED1 will be ~2.1V * 50ma or ~105mW. Power for LED2 will also be 105mW. Ditto the others... You're pulling a total power of 525mW (the "power of 5 LEDs", not the "power of one LED") in the LEDs and a total power of ~14.2 V * 50mA = 710mW, meaning 525 in the LEDs and 185mW in the resistor. |
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